1. Field of the Invention
The present invention concerns bodies of foamed thermoplastic polymers, e.g., polyolefins such as polyethylene, in forms such as sheets or planks, containing residual blowing agents which require removal for safe shipment or storage.
2. Description of the Prior Art
Thin sheets of polyolefin foams (1/4" thick and less) are useful for protective wrapping and cushioning. Such foams are disclosed in U.S. Pat. Nos. 4,694,001, 3,808,300, 3,384,090 and 3,637,458. The blowing agents used for the production of these foams include halogenated hydrocarbons, generally chlorofluorocarbons such as trichlorofluoromethane, dichlorodifluoromethane and dichlorotetrafluoroethane, and hydrocarbons, such as n-pentane, isopentane, n-butane, isobutane and propane. Chlorofluorocarbon (or CFC) blowing agents have the problems of being relatively expensive and it is believed that their release into the atmosphere is causing a decrease in the Earth's protective ozone layer. The hydrocarbon blowing agents have the disadvantage of being very flammable. It is therefore intended to devise a method of purging or removing the hydrocarbon and/or halogenated hydrocarbon blowing agent from the foam product rapidly before shipment so the customer receives a completely safe material and environmental hazards are reduced.
The foam products discussed are produced by extrusion foaming in which a thermoplastic polymer such as a polyolefin is melted and mixed, with any additives desired, in an extruder. After the components are sufficiently melted and mixed, the blowing agent(s) is injected into the extruder and mixed with the melt to form a homogenous mixture. This mixture is then cooled to the proper temperature for foaming. The foamable melt is then usually extruded under pressure through a die. Upon the release of pressure, the blowing agent volatizes to expand the polymer, forming a foamed product. In the case of sheet foam, an annular die is used and the extrudate is pulled from the die over a sizing apparatus such as a solid cylinder and then slit into one or two webs which are then wound into rolls. From the die, until the sheet is wound onto a roll, the foam usually loses about 40 to 70 percent of the blowing agent injected into the extruder. The amount dissipated during this time is dependent upon the polymer, blowing agent and foam thickness. The foam sheet, containing the remainder of the blowing agent, is wound into a roll. The problem is then to purge the foam sheet roll of the remaining blowing agent to make it safe for shipment and use, particularly if the blowing agent is a flammable material such as a hydrocarbon.
The foamable compositions of the prior art may contain permeability adjustment additives which decrease the permeability of the polyolefin to the blowing agent. The use of such additives minimizes post-extrusion collapse by making the rate at which the blowing agent leaves the cells nearly equal to the rate at which air enters the cells at or near room temperature. The use of blowing agents with relatively slow diffusion rates through polymer cell walls also helps to limit this collapse. It has been found through applicant's experimentation with prior art processes for the production of foam sheeting that if the foam is allowed to collapse too much and too fast, the foam will not fully recover to its original quality and, in fact, may be destroyed.
After a hydrocarbon-blown foam sheet is extruded and rolled up, it normally requires several weeks or even months to reduce the hydrocarbon content of the foam to a safe level if stored at or near the usual ambient temperatures (about 40.degree. F. to 120.degree. F.). This is also true of foam sheet which is at least partially unwound or stacked in sheets. It has been observed that rolled foam sheeting blown with halogenated hydrocarbon blowing agents loses the blowing agent from the outer layers of foam faster than from the inner layers. Obviously, the higher the temperature, the sooner this gas exchange (aging) will occur. However, this method of letting the blowing agent gradually diffuse from the foam requires warehousing facilities capable of storing a large volume of foam during its curing. Because the hydrocarbon diffuses so slowly from the foam, it would be relatively uneconomical to collect and combust the flammable residue. Therefore, the fumes would be released into the atmosphere, contributing to pollution such as low level smog and ozone.
U.S. Pat. No. 4,337,321 discloses methods for foaming heat foamable thermoplastic resin preforms in a series of foaming stages, each series comprising at least one stress-relieving foaming stage and at least one additional foaming stage subsequent thereto and at a higher temperature. The method is said to facilitate the production of foam sheets, slabs or planks of better quality than previously available from known single-stage foaming methods. An example of two-stage foaming of crosslinked polyethylene preforms is included. This patent deals exclusively with the problems of foaming thermoplastic resins, primarily with chemical blowing agent which decompose to release normally gaseous decomposition products, and does not discuss the problem of purging flammable blowing agents from completely foamed products, let alone the concept of rapid purging thereof.
U.S. Pat. No. 4,657,938 discloses sprayable and foamable insulating compositions comprising low molecular weight polymers including polyolefins, polyacrylates, polymethacrylates, polyisoprenes, polyacrylonitrile, polybutadiene and the like; a curative, and a blowing agent. The blowing agents can include hydrocarbons, optionally halogenated hydrocarbons. The purging of such blowing agents from the foamed compositions is not discussed.
U.S. Pat. No. 4,681,715 discloses steam-expandable alkenyl aromatic polymer compositions which are mixed with combination blowing agents which can include dichlorodifluoromethane in combination with a halogenated hydrocarbon, a hydrocarbon or an aliphatic alcohol. The compositions can be expanded by steam immediately after extrusion foaming or expanded after aging of the composition which has been impregnated with a volatile blowing agent. The object is to provide rapid expansion of the composition in steam to very low densities without adversely affecting the final properties or structure of the foams. The primary blowing agent is selected to have adequate stability to plasticize the polymer, which helps to lower the glass transition temperature of the polymer and thereby facilitate steam expansion according to the patent. The secondary blowing agent then diffuses out of the composition without adverse effects on the final foam properties. This is said to be advantageous, since other plasticizing agents such as solid or liquid compounds mixed into the polymer will permanently remain in the final cell structure. Although the patent refers in col. 4 to the aging of foam beads before exposing them to the steam, there is no discussion of the problem of complete purging of flammable blowing agents from finished foam products.
Park's U.S. Pat. Nos. 4,640,933, 4,663,361 and 4,694,027 disclose expandable polyolefin compositions and methods of preparation thereof, in which isobutane or mixtures of isobutane with other physical blowing agents are employed to expand the compositions which have been modified by the addition of a stability control agent. The compositions are said to have a high degree of dimensional stability and exhibit minimal shrinkage during curing and/or aging. The '361 patent claims expandable polymeric compositions, the '933 patent claims expanded polyolefin foams and the '027 patent claims processes for preparing polyolefin foams. The process claim recites a permeation rate through an olefin polymer resin modified with the stability control agent of greater than about 1.2 times the permeation rate of air. Since these three patents issued from related applications, their disclosures are quite similar. Col. 1 describes the process of aging or curing in which the blowing agent gradually diffuses from the closed foam cells and air gradually diffuses into the cells to replace the blowing agent. Permeability modifiers and stability control agents are discussed which are designed to slow the diffusion of volatile hydrocarbon blowing agents out of polyolefin foam cells. Col. 2 describes a problem of excessive shrinkage of foam products where butane alone is used as the blowing agent, and notes that this blowing agent can normally be utilized only in small amounts in conjunction with the more expensive halogenated hydrocarbons. Col. 2 also notes a "dramatic difference" in the permeation rates between n-butane and isobutanes from polyolefin films modified with stability control agents. This disclosure provides information regarding permeation or stability control agents and the measurement of permeability, but does not discuss the problem of the lengthy times required for complete diffusion of flammable blowing agents from foam products, let alone suggesting techniques for speeding up the process. In fact, the invention disclosed appears to be directed to decreasing the rate of diffusion of the blowing agent from the foam product so that atmospheric air will permeate the cells at least as rapidly as the blowing agent diffuses, thus minimizing shrinkage or deformation in the curing process.
U.S. Pat. No. 4,721,591 discloses crosslinked polyethylene foams prepared with specialized chemical crosslinking agents or initiators. A sheet material is heated in two stages to produce a crosslinked foam material of a microcell structure. Chemical blowing agents are used rather than hydrocarbons or the like.
Clearly improved techniques are needed for removing sufficient residual blowing agent from objects produced from foamed thermoplastic polymers that the objects produced can be stored or shipped directly without constituting fire safety hazards or other safety or environmental hazards due to the slow diffusion of such blowing agents from the foam. Preferably, the improved methods should provide for rapid purging of gaseous blowing agents from foam objects to minimize processing and storage time, and most preferably be rapid enough to permit the removal of the blowing agent to take place as part of the production line process.